Well, here is the description from Dr. Fosters & Smiths web site.
"The Spider Decorator Crab, also known as Camposcia Decorator Crab or Decorator Crab, is a member of the Majidae family of spider and decorator crabs. It decorates its brown body with sponges, shells, rocks, and other items as a method of camouflage.
It is nocturnal, hunting at night. It is not reef compatible, eating some invertebrates. It may be harmful, as well, as it harvests algae, soft corals, and invertebrates to adorn its shell.
The diet of the Spider Decorator Crab should include meaty foods such as chopped fish and clams.
Approximate Purchase Size: 1-1/2" to 3"
By saying that they hunt and eat some invertebrates I guess I jumped to saying it was a carnivore...I may be wrong.... he is an invertivore. But they do suggest feeding them meaty foods & chopped fish and I think that is the definition of carnivore.